In optical fiber manufacturing processes, fiber is drawn from a large-diameter glass structure known as a preform. Processes for making a preform include modified chemical vapor deposition (MCVD), outside vapor deposition (OVD) and vapor axial deposition (VAD). In MCVD, a hollow glass tube is collapsed inwardly to form a solid glass core rod to which cladding layers are then added. In OVD and VAD, particles are deposited on a glass core rod (also known as a seed rod or bait rod). In some types of processes for making a preform, it is important that the core rod be straight before the core rod is drawn or further elongated.
One method for straightening a glass core rod of the tubular type used in MCVD uses a rotating machine having a heating torch to soften the rod and a machine-vision feedback system to control the speed of rotation. The machine-vision feedback system measures the amount of bow in a rotating core rod and adjusts the speed of rotation to allow gravity to pull any upward bow in the core rod downwardly. While this method may be suitable for straightening the thin-walled tubes used in MCVD, it may be less suitable for straightening the solid (and thus more massive) core rods used in OVD and VAD.
Another method for straightening a glass core rod involves placing the core rod in a rotating machine, commonly referred to as a straightening lathe, and manually straightening the core rod. As a heating torch, which may be mounted on a carriage, is moved to different positions along the length of the core rod, an operator visually judges the straightness of the rotating core rod and presses a tool against portions of the rotating core rod judged to be bowed, until the operator judges the rod to be straight. After the core rod has been straightened, it may be transferred to a similar rotating machine, commonly referred to as an elongation lathe. The elongation lathe stretches or elongates the core rod while a heating torch, which may be mounted on a carriage, traverses the length of the core rod.
Some lathes of the types described above that are used to straighten or elongate core rods may include an optical measuring system comprising a laser and an optical sensor. The laser may direct a beam toward the core rod, and the optical sensor may receive the beam partially blocked by the core rod. Based on the optical sensor, the measuring system may display for the operator a measurement of the diameter or displacement of the core rod. The optical measuring system may be mounted on the same carriage as the torch.